v10.4.0

Every place a tool named a template, it pointed the caller at GitHub and
never at the catalog that serves the same template with screenshots,
metadata and a deploy button. The MCP was routing around its own front
door. Template emissions now carry the catalog detail URL alongside the
existing fields, and the offline fallback corpus moved to the commit the
catalog actually serves.

• extension_templates list results gain catalogUrl,
  https://templates.extension.dev/<slug>?utm_source=mcp&utm_medium=tool,
  beside repositoryUrl and downloads, which are unchanged. The source
  view and extension_add_feature's reference template carry the same
  field.
• extension_create results gain templateCatalogUrl for the template
  the project was scaffolded from, with utm_source=mcp-create so
  create-path visits stay distinguishable from browse-path visits.
• The pinned fallback template corpus moved to 52c0d871c433, the commit
  the live latest channel pointer resolves to, so the offline corpus and
  the catalog agree.